SCIENCE

"It suggests something on the order of a tribal group," Hey said in a telephone interview last week. "So few people may be a little bit surprising, but there hasn't been much information out there. To the extent that there is conflict, the conflict is with scholars' intuition."

Hey, reporting in the June issue of the journal PLoS (Public Library of Science) Biology, said he used genetic data from northern Asia and from Native American populations from the United States southward to the Andes and the Amazon basin.

He said the peak probability for time of settlement based on his calculations was 7,000 to 8,000 years ago, but "the peak is very broad" and extends back to 13,000 or 14,000 years ago, which corresponds roughly to estimates based on archaeological records.

"My analysis is consistent with other estimates," he said, even though a fierce argument rages among archaeologists who date the arrival of the first immigrants anywhere from 11,000 to 14,000 years ago, or even earlier.

-- Guy Gugliotta

Martian Meteor Seen on Film

The panoramic camera on the Mars rover Spirit was pointed east a little before dawn on March 7, 2004, when a streak of light passed from left to right low on the horizon, flaring for perhaps 15 seconds. Then it was gone.

"We actually talked about it when we saw the exposure," said Texas A&M University planetary scientist Mark T. Lemmon, in charge of the panoramic camera for the NASA Mars mission. "We thought it was either a meteor or a satellite -- maybe Viking 2, which we haven't heard from in 20 years."

But in research reported in the journal Nature last week, Lemmon joined a team of scientists led by Franck Selsis of the Lyon Astronomical Research Center in France to conclude that Spirit had quite likely captured the first-ever image of a Martian meteor.

Meteor showers can occur when a planet passes close to the orbit of a comet, which leaves dust in its wake as it courses through the heavens. A shower results from particles "grazing" the planet's atmosphere in a streak of light familiar to any sky watcher on Earth.

"We probably would have seen more if we had been looking earlier -- around 2 a.m. is best," Lemmon said. But the team was still able to show that the meteor came from the orbit of the comet Wiseman-Skiff. The team said showers probably occurred regularly whenever Mars approached the comet's orbit.

Lemmon said satellites illuminated by the sun can also provide a similar light streak, but the NASA scientists discounted all the satellites orbiting Mars, including Viking 2, launched in 1975 and shut down in 1978.

-- Guy Gugliotta